The present invention relates to an aluminum alloy foil for use as the cathode foil of an electrolytic capacitor which comprises an anode foil formed with a dielectric coating, a cathode foil having no dielectric coating and opposed to the anode foil and an electrolyte provided between the two foils.
Throughout the specification and the appended claims, the percentages are all by weight.
To give an increased capacitance to electrolytic capacitors of the type described, it has been practice to increase the capacitance of the anode foil through various improvements in the anode foil, and an aluminum foil having a purity of about 99.3 to 99.8% is used as the cathode foil. Such aluminum foil is produced by casting a slab from aluminum of the above-mentioned purity by a semi-continuous process, and subjecting the slab to hot rolling, then to cold rolling and to further rolling for foil forming. However, the aluminum foil thus prepared still remains to be improved in its capacitance.
To obtain a cathode foil of increased capacitance, the aluminum foil is etched to form minute indentations in its surface uniformly and with a high density and is thereby given a larger surface area. Nevertheless, when having a purity of about 99.3 to 99.8%, the aluminum foil must be etched excessively so as to have the desired capacitance. An excess of weight reduction will then result from corrosion, permitting occurrence of corrosion pits locally in the foil and consequently reducing the capacitance and mechanical strength of the foil.